Motorola DSP56301 - Manual (Page 208)

Operating Modes: Normal, Network, and On-Demand
7 -10 DSP56301 User’s Manual
7.4 Operating Modes: Normal, Network, and On-Demand
The ESSI has three basic operating modes and several data and operation formats. These
modes are programmed via the ESSI control registers. The data and operation formats
available to the ESSI are selected when you set or clear control bits in the CRA and CRB.
These control bits are WL[2–1], MOD, SYN, FSL[1–0], FSR, FSP, CKP, and SHFD.
7.4.1 Normal/Network/On-Demand Mode Selection
To select either Normal mode or Network mode, clear or set CRB[MOD]. In Normal mode,
the ESSI sends or receives one data word per frame (per enabled receiver or transmitter). In
Network mode, 2 to 32 time slots per frame can be selected. During each frame, 0 to 32 data
words are received or transmitted (from each enabled receiver or transmitter). In either case,
the transfers are periodic.
The Normal mode typically transfers data to or from a single device. Network mode is
typically used in time division multiplexed networks of CODECs or DSPs with multiple
words per frame.
Network mode has a submode called On-Demand mode. Set the CRB[MOD] for Network
mode, and set the frame rate divider to 0 (DC = $00000) to select On-Demand mode. This
submode does not generate a periodic frame sync. A frame sync pulse is generated only when
data is available to transmit. The frame sync signal indicates the first time slot in the frame.
On-Demand mode requires that the transmit frame sync be internal (output) and the receive
frame sync be external (input). For simplex operation, Synchronous mode could be used;
however, for full-duplex operation, Asynchronous mode must be used. You can enable data
transmission that is data-driven by writing data into each TX. Although the ESSI is
double-buffered, only one word can be written to each TX, even if the transmit shift register is
empty. The receive and transmit interrupts function normally, using TDE and RDF; however,
transmit underruns are impossible for On-Demand transmission and are disabled. This mode
is useful for interfacing with codecs requiring a continuous clock.
Note: When the ESSI transmits data in On-Demand mode (that is, MOD = 1 in the CRB
and DC[4–0]=$00000 in the CRA) with WL[2–0] = 100, the transmission does not
work properly. To ensure correct operation, do not use On-Demand mode with the
WL[2–0] = 100 32-bit word length mode.